1. Field of the Invention
This present invention relates to the use of an electronically controlled humidity chamber with temperature controls to collect vapor desorption data which is used to calculate the capillary pressure of a core sample. In another aspect of the present invention, a core sample with a high capillary pressure is produced and at least one electrical property is determined. In another aspect of the present invention, the capillary pressures determined according to the present invention are used to transform capillary pressures determined from the high-pressure mercury injection method.
2. Description of the Related Art
Knowledge of capillary pressure for each specific rock/oil/water combination present in reservoir rocks is highly important for predicting potential hydrocarbon in place within a reservoir. Capillary pressure data is a measure of the interaction between fluids and the rock pore surface. The strength of capillary interaction varies with the fluid saturations, the interfacial tension between the fluids, the pore structure, and the wettability of the pore surfaces.
Capillary pressure measurements also provide basic descriptions of the reservoir rock, fluids and rock-fluid behavior. Capillary pressure data may be used to: estimate pore throat size distribution to classify hydraulic rock types; determine initial water saturation conditions; estimate water saturation, permeability, porosity, and height above free water level for reserve estimates; estimate absolute permeability; estimate seal capacity of the sealing facies; and estimate capillary pressure water saturation profiles.
Several techniques have been developed for measuring capillary pressures of core samples, including porous plate, centrifuge and mercury injection. As known by one of skill in the art, the porous plate and centrifuge methods have the advantage of being able to use reservoir fluids during the capillary pressure measurements; however, limitations on the maximum achievable capillary pressure preclude application in situations where high capillary pressure exists, such as tight gas sands. The porous plate and centrifuge methods are generally limited to capillary pressures up to about 1000 psi.
High-pressure mercury-injection can reach the necessary pressures, typically 5000 to 10,000 psi, but the use of non-reservoir fluids to compute capillary pressures produces inaccurate results and transformation is required to correct the capillary pressure data. The inaccurate results are believed to be due to the lack of a true wetting phase during testing. The test is performed on dry samples using mercury as the non-wetting phase fluid and assuming air is the wetting liquid. This requires conversion to reservoir conditions using contact angle and surface tension inputs. Additionally, the oil and gas industry lacks a consensus of standards for correcting system compressibility at high pressures resulting in water saturation/capillary pressure distribution measurement uncertainties. Finally, use of the contact angle and surface tension scaling parameters are generally not appropriate for rocks with ultra-low water saturations and high capillary pressures or rocks common to tight gas sand reservoirs.
It is also known that the vapor desorption method can be used to calculate capillary pressures. While the vapor desorption method produces accurate results at moderately high capillary pressures and moderately low water saturations, the vapor desorption measurement precision decreases at very high relative humidity (>95%) which limits the lower limit of capillary pressure to a range of approximately 1000 psi. Thus, a disadvantage of the vapor desorption technique is the inability to measure capillary pressures at high water saturations.
For the vapor desorption method, it is known that the Kelvin relationship: Pc=ln(RH/100)RT/Vm, (where: Pc is the capillary pressure, psi; RH is the relative humidity; R is the universal gas constant, 8.314 J/Mol K; T is the absolute temperature, degrees Kelvin; and Vm is the molar volume of water) can be used to compute air/brine capillary pressures for core samples. (The Kelvin relationship is known to those of skill in the art.) This is detailed in SPE Paper No. 16286, “Use of Water Vapor Desorption Data in the Determination of Capillary Pressures”. Experimentally, the core samples are allowed to reach equilibrium in a constant vapor pressure environment. As discussed in the paper, a known way to establish the constant vapor pressure environment is to use saturated solutions of salts such as BaCl2, KNO3, and K2SO4. Using the vapor pressure data for these solutions, the Kelvin capillary pressures are calculated for a range of NaCl brine compositions and a range of temperatures. The lowest humidity level shown in the paper is 0.8987 produced by a saturated solution of BaCl2 at 30° C. Applicants are not aware of salt solutions that will produce practical humidity levels below this 0.8987 level. The paper discusses the use of salt solutions to calculate capillary pressures as high as 4000 psi.
Electrical properties such as formation factor and resistivity index are often calculated at a number of varying water saturations or capillary pressures. However, because the porous plate and centrifuge methods are limited on the maximum capillary pressure, the calculation of these electrical properties has been limited to high water saturations and low capillary pressures.
Thus, there are a number of shortcomings with the prior art, including: the inability to accurately determine high capillary pressures; the inability to produce core samples having low water saturation and high capillary pressure for measurement of electrical properties; and the inability to obtain accurate capillary pressures using the high-pressure mercury injection method.